Regulations and social demands for environmental conservation are intensifying day by day and still further efficiency improvement and NOx reduction are being required today also in the field of gas turbines. As a method for increasing the efficiency of a gas turbine, it is possible to increase the gas temperature at the inlet of the turbine. In this case, however, there is an apprehension that the amount of NOx emission increases with the increase in the flame temperature in the gas turbine combustor.
There exist gas turbine combustors employing premix combustion in order to reduce the NOx emission. The premix combustion is a combustion method in which air-fuel mixture obtained by previously mixing fuel and air together (premixed gas) is supplied to the gas turbine combustor and burned. Such a gas turbine combustor employing the premix combustion comprises a burner which has a premixer for previously conducting the mixing of fuel and air and a combustion chamber which is arranged downstream of the burner to burn the air-fuel mixture. The premix combustion is effective for the NOx reduction since the flame temperature is uniformized by the premix combustion. However, the possibility of flashback (flame unexpectedly flowing back to the premixer) increases since the combustion speed increases with the increase in the air temperature or in the hydrogen content in the fuel. Thus, there is an increasing demand for a gas turbine combustor achieving both NOx emission reduction and flashback resistance.
In regard to such a gas turbine combustor achieving both NOx emission reduction and flashback resistance, Japanese Patent No. 3960166 discloses a technology of a gas turbine combustor comprising a perforated coaxial burner which includes multiple fuel nozzles and multiple air holes arranged coaxially and supplies multiple coaxial jets of fuel and air (air-fuel coaxial jets) to the combustion chamber to cause combustion. The gas turbine combustor disclosed in the Document can achieve both NOx emission reduction and flashback resistance since the gas turbine combustor is capable of rapidly mixing fuel and air together in an extremely short distance compared to gas turbine combustors employing conventional premix combustion methods. Further, while fuels of high hydrogen content and high combustion speed (coal gasification gas, coke oven gas, etc.) have been handled so far by means of diffusion combustion, the gas turbine combustor disclosed in the Document is applicable also to this type of fuels.
Japanese Patent No. 4838107 discloses a structure in which a plurality of air-fuel coaxial jets are arranged in multiple concentric circular patterns (rows) around the center of the burner. In this structure, the plurality of air-fuel coaxial jets are grouped into multiple concentric circular groups. This method, increasing and decreasing the number of coaxial jets (supplying the fuel) in regard to the radial direction according to the increase/decrease in the load on the gas turbine, is called “fuel staging”.